(1) Field of the Invention
The present invention relates to a firearm projectile or bullet, specifically a bullet for a firearm which provides for an expanding counter bore to provide decreased contact between the bullet and barrel during loading and with barrel rifling during firing.
(2) Background of the Invention
Hunting and shooting with muzzleloaders is rapidly gaining popularity as a sport. The muzzleloader is essentially a primitive rifle, shotgun, or pistol, based on designs used during the early days of America and lacking the effective range of more modem center fire rifles and the speed of reloading available to cartridge firearms. Because of their popularity, many states have adopted special muzzleloader seasons for hunting with these weapons to allow sportsmen using them (who generally have to get much closer to their targets and be more sure of their aim than those using modem cartridge rifles) to be able to effectively hunt. With the creation of these special seasons, many hunters are moving from more modern rifles to muzzleloaders to take advantage of the special season.
By their very nature, muzzleloaders are essentially primitive firearms, and for many hunters and shooters this primitive nature is part of their appeal. The weapon's decreased effective range requires the hunter to be a more effective stalker. Further, the time it takes to reload a muzzleloader generally means that the hunter gets only a single shot at a target requiring them to be sure of their aim before firing, or to track moving wounded prey. There is also polarization in muzzleloading hunting. Some wish to only utilize traditional firearms and are very interested in the nostalgia, while others are continuously modernizing the “primitive” firearm to provide for improved triggering, safety, and accuracy, while still keeping the tradition of loading powder and shot down the muzzle instead of using a cartridge to make a better firearm for the special season.
As opposed to a more modem firearm which is loaded with a cartridge at the breach, in a muzzleloader loose powder (or powder pellets) and the projectile are loaded into the barrel via the muzzle of the gun and tamped against the breach plug. Because the powder, projectile and percussion cap are separately loaded for each shot and are loaded down the muzzle, the size and shape of the projectile can be very important to insure accurate loading and shooting. To provide for a truer shot, most muzzleloaders utilize a rifled barrel where the interior surface of the barrel is grooved in a helical pattern. The upraised lands in the barrel therefore ideally will contact the bullet as it leaves the barrel imparting spin to the bullet to provide for a more stable shot.
To function most efficiently, muzzle loading firearms require a good gas seal between the propellant charge (powder) and the projectile so that propellant gases cannot escape around the projectile decreasing muzzle velocity. This is called “blowby.” Blowby can decrease muzzle velocity and in some cases can even cause the bullet to be deflected from its true path due to propellant gas leakage. In many firearms, a wad or gas check member is placed between the projectile and the powder charge to reduce blowby. The gas check usually serves as the seal and forces the bullet forward in front of the gas check during firearm discharge. Alternatively, the bullet may be wrapped in a cotton or silk wad to try and better seal the bullet itself to form the gas check. This system also helps to hold the bullet in place within the bore during transport of a loaded firearm to prevent the bullet from sliding out of the muzzle if the muzzle was pointed downward. While these systems are more traditional, they often form an inconsistent seal, being less than ideal.
While these initial sealing solutions are still in use, those looking for more modem solutions will often use sabots or gas checks attached to the rear of a bullet instead of wads or separable gas checks to provide for sealing. A sabot is generally a more modem bullet casing which surrounds the bullet. The plastic structure lies tightly between the barrel and bullet to form a seal and allow the bullet to leave the barrel without being altered by the act of loading the gun. The bullet does not contact the barrel during loading or shooting so the sabot absorbs all disfiguration.
Sabots were conventionally made of expansive packing material such as molding paper, leather or other materials, but are now almost universally made of a plastic. Plastic sabots generally serve to better seal and prevent “blowby” where propellant gases pass beside the bullet during firing because the plastic can be tightly fit to the barrel without risking damage to the bullet. Resistant plastic is used due to its low cost and its ability to distend during loading, improving the ease that the larger sabot can be rammed down the barrel. The use of plastic sabots, however, presents the problem that they almost universally leave a plastic residue in the barrel from friction against the rifle bore or lands, particularly due to burning of the plastic during firearm discharge. This residue can spoil the ballistic integrity of the barrel after only a couple of shots and requires solvent cleaners to remove. Further, where wrappers or sabots are used, such items engage both bullet and bore surrounding the bullet on the sides and rear. In these cases, the bullet is dependent on the sabot to cause the bullet to spin as the bullet itself does not engage the rifling grooves due to the sabot being on the sides. With these designs, ballistic qualities of the plastic can effect the bullet exiting the bore as the bullet is entirely dependent on the sabot for spin. Further, it is intended that these devices separate from the bullet upon the bullet leaving the barrel so as to avoid them impeding velocity.
To deal with the problems, many individuals have tried to make bullets where the outside surface of the bullet is shaped and sized to interact with the lands without needing a sabot. There are a number of these bullets including those with special rings to interact with the lands and others that include raised sections holding lubricants and the like to smooth bullet passage on loading. The concern with all of these bullets is that in an ideal situation, the bullet will have no contact with the lands when it is loaded in the barrel to make loading easier and prevent residue accumulating. At the same time, these system often have minimal contact with the lands during firing, meaning that spin is not always correctly applied to the bullet, and often still require conventional gas checks to prevent blowby. Additionally, because of the bullet body contact, the firearm is usually more difficult to load and the loading process can be much slower when a follow-up shot may be needed quickly to bring down wounded, and potentially dangerous prey.
More modern bullets utilize integral gas checks attached to the rear of the bullet. These often have the same plastic build up problem as sabots due to them having to be pushed into the barrel and being in contact with the lands to provide for a gas seal. These type of devices also often will utilize differential speeds between the front and back of the bullet to have the bullet expand to contact the rifling. This often results in minimal contact between bullet and rifling during firing. Further, as the gas check is often also in contact with the rifling, interfering contact is possible.
A more problematic issue with sabots and breakaway gas checks is from interference between the sabot or gas check and the bullet as the bullet clears the muzzle. It is intended that as the bullet leaves the barrel, the sabot or gas check will separate from the bullet, flying clear of the bullet which continues to the target. Most of the time, this is accomplished by having the sabot or gas check slow down at a significantly quicker rate than the bullet. In this way the sabot will separate from the rear of the bullet as the bullet flies clear. The problem that has been found is that unless the breakaway system works perfectly every time, the sabot can hit or interact with the bullet as it breaks away spoiling the shot. Further, if there is no breakaway because the gas check begins separate, generally there is the possibility of positioning errors in placing the gas check, also resulting in ballistic effect. This is a major problem with traditional “petaled” sabots, but can also be a problem with plastic gas checks which are connected with centralized pins or other systems. Regardless of the type of device used, interaction of the device with the bullet can alter the bullet's trajectory making it less accurate.